The invention relates generally to security systems and, more particularly, to electronic surveillance systems.
Magnetic electronic article surveillance (EAS) systems are often used to prevent unauthorized removal of articles from a protected area, such as a library or retail store. A conventional EAS system usually includes an interrogation zone located near an exit of the protected area, markers or tags attached to the articles to be protected, and a device to sensitize (activate) or desensitize (deactivate) the markers or tags. Such EAS systems detect the presence of a sensitized marker within the interrogation zone and perform an appropriate security action, such as sounding an audible alarm or locking an exit gate. To allow authorized removal of articles from the protected area, authorized personnel desensitize the marker using the EAS system.
An EAS marker typically has a signal producing layer that, when interrogated by a proper magnetic field, emits a signal detectable by the EAS system. Markers of a xe2x80x9cdual statusxe2x80x9d type, i.e., markers capable of being sensitized and desensitized, also have a signal blocking layer that can be selectively activated and deactivated. When the signal blocking layer is activated, it effectively prevents the signal producing layer from providing a signal that is detectable by an EAS detection system. Authorized personnel typically activate and deactivate a magnetic EAS marker by passing the marker near a magnetic field produced by the EAS system. The EAS system may include, for example, an array of magnets or an electric coil that produces a magnetic field of a desired intensity to change the state of the signal blocking layer of the marker. Many conventional EAS systems make use of a high voltage power supply and a tuned resistor-capacitor-inductor (RCL) circuit for controlling the magnetic field when sensitizing and desensitizing markers.
In general, the invention is directed to techniques for creating and controlling a magnetic field for use with electronic article surveillance (EAS) markers. Unlike conventional systems that may incorporate an RCL circuit or other circuit for generating the magnetic field, the techniques make use of current switching devices to generate a signal having one or more current pulses for creating the magnetic field.
In one embodiment, the invention is directed to an electronic article surveillance (EAS) system having a coil to create a magnetic field for interacting with an electronic marker and a drive unit to output a signal having one or more current pulses for energizing the coil. A programmable processor within the EAS system controls the drive unit to generate the output signal according to a desired profile. To generate the output signal, the processor selectively activates electronic current switching devices within the drive unit.
By selectively activating and deactivating the current switching devices, the processor can direct the drive unit to generate the output signal according to a desired profile having a number of current pulses of different amplitudes and polarity. The drive unit may advantageously generate the output signal such that the rate of change of the current (di/dt) is substantially constant and, therefore, the current increases or decreases at substantially constant rates. Furthermore, the frequency of the pulses need not be fixed and can be readily controlled by the processor. These features have many advantages including improved marker detection over conventional systems in which the rate of change of the coil current typically follows a sinusoidal or other non-linear profile.
In addition, the programmable processor within the EAS system may dynamically adjust the current pulses of the output signal based on a number of factors including one or more configuration parameters set by a user, a type of article to which the marker is affixed, a sensed drive voltage and intensities of previously generated magnetic fields. In this manner, the EAS system is able to generate magnetic fields suitable for a variety of articles ranging from clothing to books to magnetically-recorded videotapes, and can compensate for effects of the surrounding environment or manufacturing variability.
In another embodiment, the invention is directed to a method including generating a signal having one or more current pulses by selectively activating and deactivating current switching devices, and driving the signal through a coil to generate a magnetic field for interacting with an electronic marker. The method may further include determining a profile for the current pulses of the signal, and selectively activating and deactivating the current switching devices according to the profile.
In another embodiment, the invention is directed to a computer-readable medium containing instructions. The instructions cause a programmable processor to calculate a target intensity for a magnetic field, and activate and deactivate a set of current switching devices to drive a pulse of current through a coil to create the magnetic field based on the target intensity.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the description, the drawings, and the claims.